1. FIELD OF THE INVENTION
This invention relates to a process for removing nitrogen oxides and sulfur oxides from a flue gas containing the nitrogen oxides and sulfur oxides.
2. DESCRIPTION OF THE PRIOR ART
Combustion apparatuses such as power plant boilers, etc. and internal combustion engines such as automobile engines, etc. have a high combustion temperature, and therefore nitrogen contained in fuel and in combustion air reacts with oxygen to form nitrogen oxides, mostly nitrogen monoxide. The formed nitrogen oxides are exhausted into the atmosphere together with flue gas. Nitrogen monoxide and nitrogen dioxide are also frequently discharged into the atmosphere from plants where nitric acid is handled.
From combustion apparatus such as the power plant boilers, etc. sulfur contained in the fuel is combusted and exhausted as sulfur oxides together with fuel gas.
These nitrogen oxides and sulfur oxides, once discharged into the atmosphere, give very harmful influences upon human bodies as well as animals and vegetables, and many processes have been so far proposed for removing these nitrogen oxides and sulfur oxides from the flue gas.
For example, absorption by an alkaline solution, washing by a slurry containing magnesium hydroxide, etc. have been proposed to remove nitrogen oxides from a flue gas from power plant boiler, etc., but these prior art processes have many problems such as failure to attain a sufficient percent removal, troublesome post-treatment of the absorbing or washing solution, or complicatedness of the process.
Furthermore, another process for removing nitrogen oxides from the flue gas has been proposed, which comprises contacting the flue gas with single activated carbon or activated carbon impregnated with vanadium oxide in the presence of ammonia, thereby removing the nitrogen oxides from the flue gas, but the prior art process has a low percent removal of nitrogen oxides, and consequently it is difficult to put the process in practice.
In addition, the so-called ammonia reduction process is now put in practice in various places, which comprises contacting the flue gas with ammonia in the presence of a catalyst such as vanadium pentoxide, iron oxide, or copper oxide at temperatures in excess of 300.degree. C., thereby reducing nitrogen oxides to nitrogen. The prior art ammonia reduction process has such characteristics that the reaction product is harmless nitrogen, and washing, discharge of waste water, etc. are not required at all, and consequently the relevant treating steps can be saved. However, if dusts are contained in the flue gas, the dusts are liable to accumulate on the catalyst, and a continuous operation is impossible to conduct. Thus, it is necessary in the prior art ammonium reduction processes to remove the dusts from the flue gas in advance.
To remove dusts, it is necessary to pass the flue gas through an electrostatic dust collector, but a hot flue gas (e.g., over 300.degree. C.) cannot be treated in the electrostatic dust collector. Thus, after the flue gas has been cooled (e.g., to about 150.degree. C.), it must be passed through the electrostatic dust collector. However, if the flue gas temperature is low, percent reduction of nitrogen oxides is considerably lowered. This is a great disadvantage of said prior art process. To overcome the disadvantages, it is necessary to pass the flue gas through an electrostatic dust collector and heat the flue gas of low temperature to 350.degree.-450.degree. C. before the removal of nitrogen oxides, which increases the necessary facility cost and running cost.
Especially where both nitrogen oxides and sulfur oxides are discharged together, for example, from power plant boilers, etc., no practical process is yet available for removing these oxides at the same time with less expenses but high efficiency, and a development for such a process has been keenly desired.